Striving to find the best move

Black is ahead in this game, as result of a previous combination exploiting a pin on the King, and winning the exchange ( Rook for Bishop ).

I played a good move ( the third best !) , but the best move and combination is why practicing tactics, calculation and visualisation is so important, as it wins games much quicker.

The question I need to ask myself is why I failed to see/find this, since the key part of it was something I had already used. I am sure time was a factor here, ie lack of, but still the idea is transferable, so should have been at the front of my mind.

Black to play and win